In the oil and gas industry, it is beneficial to know pertinent properties of materials encountered down hole whether during initial drilling or thereafter. Well drilling and operation processes often require specifically formulated compositions, including cements and other compositions. It is beneficial to know how materials, cements and other compositions are affected by temperature, pressure, and time. Test cells are used for measuring volumetric expansion and shrinkage of materials including shale, cements, drilling fluids, and other material samples under high pressure and high temperature conditions similar to those found in a downhole environment.
A conventional test cell for measuring expansion and contraction of cement compositions uses a linear variable differential transformer, LVDT, to determine expansion or contraction of materials in a test cell. A magnetic core translates within an array of electrical coils. The translation of the magnetic core produces an electrical differential between the coils indicating the location of the core. Such differential is quantified to determine volume change.
An expansion/shrinkage cell is disclosed in U.S. Pat. No. 6,918,292 issued to Boncan and Bray in 2005. The cell is comprised of a mold having two wall sections attached to each other using springs and seals that allow a limited degree of expansion and contraction. One of the wall sections is attached to a base, while the other is left free to move with the expansion or contraction of the sample material. A cement sample is placed in the mold and allowed to cure. A linear displacement transducer measures the movement of the first and second wall sections relative to one another in response to volumetric changes in the cement.
U.S. Pat. No. 7,240,545 issued to Jennings in 2007 discloses a cement expansion and shrinkage cell that employs a moveable piston rod, which separates two chambers, to measure expansion and contraction. A digital measuring device, such as a linear variable differential transformer, is connected to the piston to measure linear movement of the piston as the sample expands or contracts.
Chandler Engineering markets a cement analyzer identified as Model 4268ES Cement Expansion & Shrinkage Option. The system measures change in volume of a cement sample using a diaphragm and displacement piston combined with a linear variable differential transformer.
As can be determined from the above prior art, test cells have traditionally used linear variable differential transformers to measure the volumetric change in the sample. However, this type of sensor has several disadvantages. To function, LVDTs require that the magnetic core be physically attached to the device that moves inside the pressurized test cell with the expansion or contraction of the sample material. Furthermore, the magnetic coil must be able to slide freely inside the test cell with coming into contact with it. This puts major constraints on the configurations of test cells using this type of sensor. Additionally, this type of sensor may be affected by external electromagnetic disturbances and has limited range and precision.